Heretofore, there have widely been used ultrasonic diagnosing apparatus for diagnosing arteriosclerosis and performing a preoperative diagnosis and a postoperative check for coronary intervention using a dilatation catheter or a high-functionality catheter such as a stent or the like.
One example of ultrasonic diagnosing apparatus discussed below is intravascular ultrasound (IVUS) diagnosing apparatus. Generally, intravascular ultrasound diagnosing apparatus are constructed to include an ultrasonic probe that makes radial scans in an artery of the patient and receives an echo (reflected wave) reflected from a reflecting object in the artery. The echo signal is amplified and detected to convert the echo intensity into an image signal on a gray scale for thereby displaying a B-mode image.
B-mode images on the gray scale can display a large lipid in plaque deposited in the blood vessel. However, it is difficult for B-mode images to display a small lipid that may be present in an initial phase of plaque growth.
Since the rupture of vulnerable plaque in an artery is considered to be responsible for acute coronary syndromes such as an acute myocardial infarction, it is clinically desirable to diagnose plaque with a relatively high degree of accuracy. Specifically, when plaque in a blood vessel ruptures, the lipid contained in the plaque blows out into the blood vessel, leading to acute coronary syndromes. Therefore, an indication of the amount of lipid contained in plaque can serve as an important diagnostic marker.
Consequently, it is desirable in the art to develop ultrasonic diagnosing apparatus capable of sufficiently displaying tissue characters and allowing the user to easily determine whether or not plaque is relatively lipid-rich.
Efforts have been made with respect to ultrasonic diagnosing apparatus to increase the frequency of a transmitted ultrasonic signal in order to increase the resolution of the B-mode image or to analyze an RF signal obtained by receiving a reflected echo for tissue characterization. For example, a ROI (Region of Interest) is established in an analytic section, some parameters are calculated from the spectrum of an RF signal in the ROI, and tissue characters are displayed by a multivariable analysis.
It might be possible to attempt to display a smaller lipid by increasing the frequency for higher resolution. However, with this possibility, the range that can be diagnosed is limited because the depth that the ultrasonic wave can reach is reduced. Ultrasonic probes that are commercially available at present emit ultrasonic waves at a frequency of about 40 MHz and it is not clear at present whether high-frequency ultrasonic probes can generally be used for tissue characterization in blood vessels.
The process of analyzing an RF signal for tissue characterization is still under development at present and established procedures are not yet available. The process of displaying tissue characters by way of a multivariable analysis requires time-consuming calculations because the number of parameters involved is quite large as is the amount of analyzed data, and so this requires calibration of each ultrasonic probe to be used, a task not easy to perform.